Astronomy, of Geology, of Physical Optics, Thermotics and other sciences. But the discovery of causes, and of the essential forms of qualities, is a triumph reserved for the later stages of each Science, when the knowledge of the laws of phenomena has already made great progress. It was not to be expected that Aristotle would discern this truth, when, as yet, there was no Science extant in which it had been exemplified. Yet in Astronomy, the theory of epicycles and excentrics had immense value, and even has still, as representing the laws of phenomena; while the attempt to find in it, as Aristotle wished to do, the ultimate causes of the motions of the universe, could only mislead. The Aristotelian maxim, which sounds so plausible, and has been so generally accepted, that "to know truly is to know the causes of things," is a bad guide in scientific research. Instead of it we might substitute this: that "though we may aspire to know at last why things are, we must be content for a long time with knowing how they are."
10. Hence if we are asked whether Plato or Aristotle had the truer views of the nature and property of Science, we must give the preference to Plato; for though his notion of a real Intelligible World, of which the Visible world was a fleeting and changeable shadow, was extravagant, yet it led him to seek to determine the forms of the Intelligible Things, which are really the laws of visible phenomena; while Aristotle was led to pass lightly over such laws, because they did not at once reveal the causes which produced the phenomena.
11. Aristotle, throughout his works, takes numerous occasions to argue against Plato's doctrine of Ideas. Yet these Ideas, so far as they were the Intelligible Forms of Visible Things, were really fit objects of philosophical research; and the search after them had a powerful influence in promoting the progress of Science. And we may see in the effect of this search the answer to many of Aristotle's strongest arguments. For instance, Aristotle says that Plato, by way of explaining things, adds to them as many Ideas, and that this is just as if a man having to reckon a large number, were to begin by adding to it another large number. It is plain that to this we may reply, that the adopting the Ideas of Cycles, along with the motions of the Planets, does really explain the motions; and that the Cycles are not simply added to the phenomena, but include and supersede the phenomena: a finite number of Cycles include and represent an infinite number of separate phenomena.
To Aristotle's argument that Ideas cannot be the Causes or Principles of Things, we should reply, that though they cannot be this, they may nevertheless be, and must be, the Conditions and Principles of our Knowledge, which is what we want them to be.
I have given an account of the main features of Aristotle's philosophy, so far as it concerns the Physical Sciences, in the History of the Inductive Sciences, Book I.
CHAPTER VI.
The Later Greeks.
Thus while Plato was disposed to seek the essence of our knowledge in Ideas alone, Aristotle, slighting this source of truth, looked to Experience as the beginning of Science; and he attempted to obtain, by division and deduction, all that Experience did not immediately supply. And thus, with these two great names, began that struggle of opposite opinions which has ever since that time agitated the speculative world, as men have urged the claims of Ideas or of Experience to our respect, and as alternately each of these elements of knowledge has been elevated above its due place, while the other has been unduly depressed. We shall see the successive turns of this balanced struggle in the remaining portions of this review.
But we may observe that practically the influence of Plato predominated rather than that of Aristotle, in the remaining part of the history of ancient philosophy. It was, indeed, an habitual subject of dispute among men of letters, whether the sources of true knowledge are to be found in the Senses or in the Mind; the Epicureans taking one side of this alternative, and the Academics another, while the Stoics in a certain manner included both elements in their view. But none of these sects showed their persuasion that the materials of knowledge were to be found in the domain of Sense, by seeking them there. No one appears to have thought of following the example of Aristotle, and gathering together a store of observed facts. We may except, perhaps, assertions belonging to some provinces of Natural History, which were collected by various writers: but in these, the mixed character of the statements, the want of discrimination in the estimate of evidence, the credulity and love of the marvellous which the authors for the most part displayed, showed that instead of improving upon the example of Aristotle, they were wandering further and further from the path of real knowledge. And while they thus collected, with so little judgment, such statements as offered themselves, it hardly appears to have occurred to any one to enlarge the stores of observation by the aid of experiment; and to learn what the laws of nature were, by trying what were their results in particular cases. They used no instruments for obtaining an insight into the constitution of the universe, except logical distinctions and discussions; and proceeded as if the phenomena familiar to their predecessors must contain all that was needed as a basis for natural philosophy. By thus contenting themselves with the facts which the earlier philosophers had contemplated, they were led also to confine themselves to the ideas which those philosophers had put forth. For all the most remarkable alternatives of hypothesis, so far as they could be constructed with a slight and common knowledge of phenomena, had been promulgated by the acute and profound thinkers who gave the first impulse to philosophy: and it was not given to man to add much to the original inventions of their minds till he had undergone anew a long discipline of observation, and of thought employed upon observation. Thus the later authors of the Greek Schools became little better than commentators on the earlier; and the commonplaces with which the different schools carried on their debates—the constantly recurring argument, with its known attendant answer—the distinctions drawn finer and finer and leading to nothing—render the speculations of those times a scholastic philosophy, in the same sense in which we employ the term when we speak of the labours of the middle ages. It will be understood that I now refer to that which is here my subject, the opinions concerning our knowledge of nature, and the methods in use for the purpose of obtaining such knowledge. Whether the moral speculations of the ancient world were of the same stationary kind, going their round in a limited circle, like their metaphysics and physics, must be considered on some other occasion.[33]
Mr. Grote, in his very interesting discussion of Socrates's teaching, notices also[34] the teaching of Hippocrates, which he conceives to have in one respect the same tendency as the philosophy of Socrates; namely, to turn away from the vague aggregate of doctrines and guesses which constituted the Physical Philosophy of that time, and to pursue instead a special and more practical course of inquiry: Hippocrates selecting Medicine and Socrates selecting Ethics. By this limitation of their subject, they avoided some of the errors of their predecessors. For, as Mr. Grote has also remarked, "the earlier speculators, Anaxagoras, Empedocles, Democritus, the Pythagoreans, all had still present to their minds the vast and undivided problems which have been transmitted down from the old poets; bending their minds to the invention of some system which would explain them all at once, or assist the imagination in conceiving both how the Kosmos first began and how it continued to move on." There could be no better remedy for this ambitious error of the human mind than to have a definite subject of study, such as the diseases and the health of the human body. Accordingly, we see that the study of medicine did draw its cultivators away from this ancient but unprofitable field. Hippocrates[35] condemns those who, as Empedocles, set themselves to make out what man was from the beginning, how he began first to exist, and in what manner he was constructed. This is, he says, no part of medicine. In like manner he blames and refutes those who make some simple element, Hot, or Cold, or Moist, or Dry, the cause of diseases, and give medical precepts professing to be founded on this hypothesis.
These passages are marked by the prudence which practical study suggests to a calm and clear-sighted man. They can hardly be said to have opened the way to a Science of Medicine; for in the sense in which we here use the word Science, namely, a collection of general truths inferred
